Drilling a hole in a structure often provides certain benefits and advantages. For example, drilling a hole in a structure, such as an airfoil, provides a means for cooling the airfoil. Specifically, airfoils, such as blades and vanes, within gas turbine engines are exposed to high temperature combustion gases, thereby requiring a method for cooling the airfoil. One such cooling method includes creating holes within the airfoil and passing pressurized air therethrough. As the airfoil rotates through the combustion gases, the pressurized air passes through the interior of the airfoil and exits the cooling holes. Depending upon the configuration of the hole, a portion of the pressurized air may pass over the exterior of the airfoil, thereby creating a film of air between the airfoil and the combustion gases. This method is often referred to as film cooling.
Film cooling efficiency is a function of the relative size of the cooling holes. Specifically, film cooling efficiency increases as the size of the holes more closely resemble each other. The manufacturing process used to drill the cooling holes, therefore, must be capable of producing such holes with sufficient accuracy and repeatability. The two methods currently used to manufacture cooling holes include electro-discharge machining (EDM) and laser drilling. EDM is a process wherein an electrode contacts a structure that is typically immersed in a dielectric fluid, thereby causing a spark and erosion at the point of contact. Although the EDM process produces very accurate holes, this process is typically slow and consumes electrodes, thereby increasing set-up time and material costs. Based upon these two characteristics, EDM is often regarded as an expensive method for producing cooling holes.
Laser drilling is typically a less expensive alternative for producing cooling holes and currently includes the use of either an unmodulated pulsed laser beam or a modulated pulsed laser beam. An unmodulated pulsed laser beam (hereinafter referred to as "unmodulated beam") typically used to laser drill holes has a pulse width of about 0.1 milliseconds (msec) to about 10 msec and a peak intensity on the order of about 1.times.10.sup.6 W/cm.sup.2 to about 10.times.10.sup.6 W/cm.sup.2. A modulated pulsed laser beam (hereinafter referred to as "modulated beam") used for the same purpose, typically has a pulse width of about 1 nanosecond (nsec) to about 500 nsec and a peak intensity greater than 1.times.10.sup.8 W/cm.sup.2. For the purposes of this invention, an unmodulated beam and a modulated beam shall be defined in respect to each other. Specifically, a modulated beam shall be defined as having a shorter pulse width and higher peak intensity in comparison to an unmodulated beam, regardless of the pulse width and peak intensity of the unmodulated beam.
When using a modulated beam, such as a beam having a 100 nsec pulse width and a 1.0.times.10.sup.9 W/cm.sup.2 intensity, to drill a hole in an airfoil, the modulated beam contacts the airfoil and vaporizes a majority of the material. The modulated beam produces a hole having a typically circular cross section because the material vaporizes rather than boils. Creating the vapor, however, leads to the formation of re-solidified vapor within and/or around the hole. The use of a high intensity beam, such as a modulated beam, also has the potential of creating plasma shielding, which occurs when the intensity of the beam is too high. Upon contacting the surface of the airfoil, the surface ionizes and a plasma layer is created, thereby shielding the internal surface of the hole from additional laser drilling.
When using an unmodulated beam, such as a beam having a 0.5 msec pulse width and a 3.0.times.10.sup.6 W/cm.sup.2 intensity, to drill a hole in an airfoil, the unmodulated beam contacts the airfoil and melts the material. The molten material escapes the hole primarily in the form of melt droplets but relatively small amounts of material may also exist in the form of vapor. Removing the material in the form of melt droplets, as opposed to vaporizing the material, reduces the amount of re-solidified vapor that forms in the upper portions of the hole and around its entrance. Re-solidified vapor is also referred to as burr. Moreover, when a beam exits a laser, an air stream usually surrounds and/or accompanies the beam so as to prevent any melt or vapor from splashing onto an optical portion of the laser. This air stream, however, often prevents the vapor from escaping the hole, thereby causing re-solidified vapor to form in and around the hole. Hence, utilizing an unmodulated beam minimizes the amount of the material transformed to vapor, thereby preventing re-solidified vapor from forming.
Using an unmodulated beam, however, often creates a boiling reaction between the unmodulated beam and the material. Specifically, the relatively long contact time between the unmodulated beam and the material causes the material to melt and often boil, thereby creating a hole with non-circular cross section. Furthermore, this boiling reaction tends to occur randomly, thereby reducing hole-to-hole uniformity. This melting reaction may also cause the formation of re-cast (i.e., re-melt), which is molten material that re-solidifies and adheres to the internal surface of the hole. Upon adhering to the internal surface, the re-cast behaves mechanically similar to the parent material but has a materially different crystalline structure compared to the parent material. Somewhere during the re-cast formation process, cracks may form, thereby producing undesirable mechanical properties within the airfoil.
What is needed is a method and apparatus that repeatedly produces holes with consistent dimensions in a structure while maintaining its mechanical integrity.